One scenic line I had the chance to travel on was from Nice to Cuneo, an unlikely trans-alpine route through the 8km Tende tunnel in the Alpes Maritimes.
The line runs close to the France-Italy border the whole way, with a connecting line coming up from Ventimiglia joining at Breil sur Roya.
The whole area, like Nice, used to be part of the Savoy/Sardinian kingdom until mostly ceded to France in 1860 (Tende itself only in 1947).
The steep valleys on the French side (Mercantour National Park) topped with ancient villages are spectacular.
When I did the trip there were convenient SNCF through trains (ancient DMUs) from Nice, but these days they seem to terminate at Breil with the through trains coming from Ventimiglia.
Cuneo (looked unsuccessfully for the mouse
) is on the flat Piedmont plain with trains onwards to Turin via Savigliano, home of the Pendolino.
The border station at Limone on the Italian side is the only place I have been accosted by police for taking photos in a prohibited area.
This was before EU free movement and apparently smuggling across the border was rife at the time.
Another highlight was when I was invited into the SNCF DMU cab by the driver who had just returned from a tour to Kyle of Lochalsh and was keen to tell me about it!